1. Field of the Invention
The present invention relates to an exhaust emission control device and method for an internal combustion engine, including an exhaust passage, a NOx catalyst disposed in the exhaust passage for reducing trapped NOx in a reducing atmosphere, and a catalyst which is disposed at a location upstream of the NOx catalyst and has an oxidation function, and an engine control unit, the engine the exhaust emission control device and method and the engine control unit causing reducing agent to be supplied to the upstream side of the catalyst, so as to cause the NOx catalyst to carry out a NOx reducing operation.
2. Description of the Related Art
Conventionally, as an exhaust emission control device of this kind, the present assignee has already proposed an exhaust emission control device disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2006-207487. This exhaust emission control device is applied to a diesel engine that has a three-way catalyst and a NOx catalyst provided in an exhaust passage at respective locations from upstream to downstream in the mentioned order. The three-way catalyst purifies i.e. decreases exhaust emissions by an oxidizing action and a reducing action. The NOx catalyst traps NOx (nitrogen oxide) contained in a lean atmosphere of exhaust gases, and performs reduction of the trapped NOx when a rich atmosphere of exhaust gases is supplied, thereby purifying the exhaust gases. Further, the exhaust emission control device includes an air-fuel ratio sensor disposed in the exhaust passage at a location upstream of the three-way catalyst, for detecting the air-fuel ratio of exhaust gases.
Normally, the diesel engine is operated under lean conditions, and during the lean operation of the engine, NOx contained in exhaust gases pass through the three-way catalyst to be trapped by the NOx catalyst. During the lean operation, the exhaust emission control device calculates an amount S_QNOx of trapped NOx, which is the total amount of NOx trapped in the NOx catalyst, and calculates a reference value S_QNOxREF for use in comparison with the amount S_QNOx of trapped NOx, based on the amount S_QNOx of trapped NOx, the temperature of the three-way catalyst, and the space velocity of exhaust gases. When S_QNOx≧S_QNOxREF holds, to supply reducing agent (HC and CO) to the NOx catalyst to thereby reduce NOx trapped in the NOx catalyst, rich spike control is carried out in which the air-fuel ratio of a mixture is controlled to a richer value than a stoichiometric air-fuel ratio.
During the rich spike control, the total amount of reducing agent supplied to the NOx catalyst is calculated as a cumulative value S_QDA of the amount of reducing agent, based on a signal indicative of the air-fuel ratio detected by the air-fuel ratio sensor, and a reference value S_QDAREF for comparison with the cumulative value S_QDA is calculated based on the reference value S_QNOxREF. When S_QDA>S_QDAREF holds, it is determined that all the NOx trapped in the NOx catalyst has been reduced, and the rich spike control is terminated.
According to the conventional exhaust emission control device described above, during the rich spike control, reducing agent contained in exhaust gases can be consumed by the oxidizing action of oxygen stored in the three-way catalyst. In this case, the exhaust emission control device calculates the cumulative value S_QDA of the amount of reducing agent based on the signal from the air-fuel ratio sensor upstream of the three-way catalyst, and hence the calculation accuracy of the cumulative value S_QDA is lowered due to consumption of the reducing agent by the oxygen stored in the three-way catalyst, which can cause improper timing for terminating the rich spike control. For example, when the timing for terminating the rich spike control is made earlier than the timing in which the NOx reducing operation of the NOx catalyst is terminated, the rich spike control is executed in spite of termination of the NOx reducing operation of the NOx catalyst. This causes the concentrations of HC and CO in exhaust gases to be held high, resulting in increased exhaust emissions and degraded fuel economy. Inversely, when the timing for terminating the rich spike control is earlier than the timing in which the NOx reducing operation of the NOx catalyst is terminated, NOx trapped in the NOx catalyst is not sufficiently reduced, which causes insufficient NOx trapping performance of the NOx catalyst, resulting in increased exhaust emissions.